1. Field of the Invention
The present invention relates generally to electric motor controls, and more specifically to detecting the position of a wound field rotor in a direct current motor.
2. Description of the Prior Art
Electric motors are an essential component of our industrialized society. One type of electric motor, the brushless DC (BLDC) motor, is already used in hard disk drives and many industrial applications, and its market share is growing significantly in automotive, appliance, and industrial applications.
Mechanically, BLDC motors consist of two coaxial magnetic armatures separated by an air gap. In certain types of motor, the external armature, which is called the stator, is fixed, and the internal armature, which is called the rotor, is mobile in that it rotates.
BLDC motors are a type of synchronous motor, which means that the magnetic field generated by the stator and the magnetic field generated by the rotor rotate at the same frequency. BLDC motors do not experience the “slip” that is normally seen in induction motors. As the name implies, “brushless” DC motors do not use brushes for commutation, which is the cyclic switching to produce the rotation of the magnetic field; instead, the motor is electronically commutated.
Because there is no slip between the rotation of the rotor and the stator magnetic field, it is important to know the angular position of the rotor in order to properly position the magnetic field in the stator so that it will continue to magnetically pull and push the rotor in the direction of rotation.
In a typical three-phase BLDC motor, each of three stator magnetic fields, or phases, is angularly spaced apart 120°, as shown at stator windings 24–28 in FIG. 1. Torque is produced because of magnetic interaction between the rotor magnetic field and the stator magnetic field. Torque occurs if the two fields are in the correct position with respect to each other. Peak torque occurs when the fields are ninety degrees apart, and falls off as the fields move closer toward alignment. In order to keep the motor running, the field produced by the stator coils must shift position, or commutate as the rotor moves to catch up. BLDC motors are traditionally commutated in a six-step pattern that controls power through a three-phase inverter. The inverter excites only two out of three phases at one time, leaving the third winding floating.
Commutation timing may be determined by position sensors on the rotor, or by a “sensorless” method of sensing back-electromotive force (back EMF) as the moving rotor influences the unused stator windings. If sensors are used, the position sensors or transducers can be Hall-effect magnetic sensors, resolvers, synchros, optical encoders, or similar absolute position sensors. These sensors increase the cost and the size of the motor, and special mechanical arrangements are necessary for mounting the sensors. Some of the sensors, particularly Hall sensors, may temperature sensitive, limiting their high-end operating temperature. Position sensors reduce the system reliability because they add components and wiring. In some applications, it is difficult to find a place to mount a position sensor on the rotor.
The sensorless drive is preferred because it reduces cost and complexity of the drive system. This method detects the position of the rotor by sensing back EMF zero-crossing events, which can only be done when the rotor is rotating. This is because back EMF in the stator windings are produced by a moving rotor's magnetic field in relation to the stator winding that is being sensed. When the magnetic field in the rotor is stationary, as it is the instant before the motor is started, traditional back EMF signals cannot be used to detect the rotor position.
If a BLDC motor does not have sensors to detect the position of a stationary rotor before start up, the rotor may be pre-positioned to a known position during a first phase of a start up process. The rotor is moved to a known position by energizing the stator winding in a particular way. Once pre-positioned, a starting ramp table is used to provide a special commutation rate to accelerate the rotor until it is possible to detect back EMF voltage in the floating winding to establish a commutation switching timing in the synchronous mode.
In some BLDC motors the magnetic field of the internal rotor is produced by a permanent magnet that is part of the rotor. In motors referred to as a “wound rotor,” or “wound field” BLDC motor, the rotor magnetic field is created by a current in a winding in the rotor.
There is a mechanical similarity between motors and generators, and a relationship between mechanical and electrical energy. A machine is classified as a motor if it converts electrical energy to mechanical form, and a machine acts as a generator if it converts mechanical energy from a prime mover to an electrical form. For example, an automotive alternator converts mechanical energy from the car engine to electricity for charging the battery and operating other accessories.
The relationship between electric and magnetic fields and the physical similarities between generators and motors makes it possible for the same mechanical machine to operate as either a generator or a motor. For example, an ordinary automobile alternator that commonly generates electrical energy may be operated as a wound field brushless motor by properly switching magnetic fields in the stator while producing a constant magnetic field in the wound rotor. Operating an alternator as a motor may be useful in starting the vehicle engine, or in assisting the engine by adding extra torque to the engine. If the alternator could perform double duty and also provide the starting torque to the engine, automobile designers could eliminate the starter motor to reduce weight, cost, and complexity.
In some applications, such as starting an automobile engine, the pre-positioning method of starting a BLDC motor is not practical because reverse rotor rotation is not allowed, and pre-positioning requires reverse rotation about 50% of the time due to the random position of the rotor when the motor stops. The position of the rotor must be known to initially apply maximum torque and always start the rotor in the forward direction.
Therefore, it should be apparent that there is a need for an improved method and system that does not use added sensors for determining the position of a wound field rotor when the rotor is stationary before starting, or otherwise turning slowly and unable to produce back EMF signals for sensing on a stator winding.